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Mary Gilligan was likely born 1820 or earlier. Clontuskert RC records start in 1827 so Mary would have been born before records are available locally. There were a good number of Gilligans in East Galway in the 1855 Griffiths. Mary may have been from Sligo but the surname was certainly present in the East Galway area.
You may want to add John and Mary to our XO Chronicles site https://irelandxo.com/ireland-xo/history-and-genealogy/ancestor-database